Besides searching the National Archives, this site also has a comprehensive list of helpful websites located under "Military Resources on other web sites." You may also order copies of military records here.
Although this site is run by a for-profit company, MySlaveAncestors.com offers tips on getting started with African American genealogy as well as dates and background concerning the census, slave schedules, and property records.
The largest genealogy organization in the world, this database is published by the Church of Jesus Christ of Latter-day Saints. Obtain a free account, search countless records including the census, and connect with other family trees.
These general databases are a great place to start your research, especially if you don't quite know what you're looking for.